fs: allow mount namespace fd

fs: allow mount namespace fd

We already allow a mount namespace id, enable mount namespace file
descriptors as well.

Link: https://lore.kernel.org/r/20240719-work-mount-namespace-v1-2-834113cab0d2@kernel.org
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>